docs: def getPrefilter(self,chn):
          """
          Returns the prefilter of signal chn ("HP:0.1Hz", "LP:75Hz N:50Hz", etc.)
  
          Parameters
          ----------
          chn : int
              channel number
  
          Examples
          --------
          >>> import pyedflib
          >>> f = pyedflib.data.test_generator()
          >>> f.getPrefilter(0)==''
          True
          >>> f._close()
          >>> del f
  
          """
          if 0 <= chn < self.signals_in_file:
              return self._convert_string(self.prefilter(chn).rstrip())
          else:
              return self._convert_string('')